Use nine-patch resource for drawing Aura overlay scrollbar thumb.

ui/native_theme/native_theme_aura.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_AURA_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_AURA_H_
 
 #include "base/macros.h"
 #include "ui/native_theme/native_theme_base.h"
 
@@ skipping 35 matching lines @@
                            Part part,
                            State state,
                            const gfx::Rect& rect,
                            ScrollbarOverlayColorTheme theme) const override;
   void PaintScrollbarCorner(cc::PaintCanvas* canvas,
                             State state,
                             const gfx::Rect& rect) const override;
   gfx::Size GetPartSize(Part part,
                         State state,
                         const ExtraParams& extra) const override;
+  bool SupportsNinePatch(Part part) const override;
+  gfx::Size GetNinePatchCanvasSize(Part part) const override;
+  gfx::Rect GetNinePatchAperture(Part part) const override;
 
  private:
   bool use_overlay_scrollbars_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAura);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_AURA_H_